Please note this document is a working draft.
Cities have used urban green spaces as a tool to enhance the health of residents, increase biodiversity, and mitigate the effects of climate change. Research has shown that urban green space can alleviate urban-heat-island effects, improve air quality, and encourage outdoor socialization and physical activity.1 However, urban green spaces are more frequently supported in higher-income, predominantly white neighborhoods, exacerbating inequity in health.2 Several U.S. cities, including Denver, have proposed goals to equitably increase green space in their cities.
Often, urban planners and residents use turf and exotic plants to expand green space, which are are not ideal for pollinator health and demand high amounts of water, which may not be sustainable in the mountain west, a region expected to experience more severe drought due to climate change. While a growing body of research suggests exposure to urban green space can improve human health and prevent premature mortality,1,3 little is known about the potential impact of native vegetation on human health in cities in the U.S. mountain west, a region with a growing population vulnerable to the impacts of climate change.
We aim to estimate the number of premature deaths that would be prevented by the implementation of various policy scenarios adding green space, including native vegetation, to the City of Denver. These policy scenarios were informed by conversations with local stakeholders in advocacy, research, and governance.
We measured existing green space in the City of Denver by the normalized difference vegetation index (NDVI) measured by the Landsat-8 satellite at a spatial resolution of 30 square meters. To define native-plants greenness, we measured the NDVI of several places in Denver (places described in more detail here) comprised of various levels of native and adapted plants during cloud-free summer (May, June, July, August) days over five years (2016-2021). Over these days in these areas, the mean NDVI was as low as 0.35 at the Denver Botanic Gardens Green Roof and as high as 0.67 in City Park, which is 0% native and consists of more watered turf. A plot in Denver Botanic Gardens that is 100% native plants had a mean NDVI of 0.54 over these days.
| Place type | Place name | NDVI, mean | NDVI, 25th-ile | NDVI, 50th-ile | NDVI, 75th-ile |
|---|---|---|---|---|---|
| high diversity | Chatfield Meadow Restoration | 0.48 | 0.47 | 0.50 | 0.50 |
| high diversity | Chatfield Prairie Garden | 0.37 | 0.35 | 0.37 | 0.37 |
| high diversity | City Park Greenhouses | 0.31 | 0.27 | 0.28 | 0.28 |
| high diversity | Denver Botanic Gardens Green Roof | 0.34 | 0.34 | 0.36 | 0.36 |
| high diversity | Kenderick Lake Xeriscape Garden | 0.40 | 0.36 | 0.37 | 0.37 |
| native spectrum | Denver Botanic Gardens, 100% Native | 0.54 | 0.52 | 0.55 | 0.55 |
| native spectrum | Plains Conservation Center, 100% Native | 0.16 | 0.23 | 0.26 | 0.26 |
| native spectrum | Green Mountain Park, 85% Native | 0.42 | 0.36 | 0.40 | 0.40 |
| native spectrum | Hogback along C-470, 75% Native | 0.36 | 0.32 | 0.35 | 0.35 |
| native spectrum | Suburban Open Space 1, Chatfield H.S., 50% Native | 0.40 | 0.37 | 0.41 | 0.41 |
| native spectrum | Suburban Open Space 2, Columbine Hills Church, 30% Native | 0.36 | 0.26 | 0.31 | 0.31 |
| native spectrum | Denver Botanic Gardens at Chatfield, 10% Native | 0.53 | 0.40 | 0.54 | 0.54 |
| native spectrum | City Park, 0% Native | 0.67 | 0.67 | 0.70 | 0.70 |
To acknowledge the lack of precision in these NDVI estimates and the range of NDVI levels that may correspond to a native-plants intervention, we defined NDVI using the mean of the Denver Botanic Gardens as as the primary analysis and that of the Denver Botanic Gardens Green Roof as an alternative value.
We conducted ten interviews with local stakeholders who are actively engaged in advocacy, policy, and governance related to native plants and green space in Greater Denver. Following these interviews, we developed four policy scenarios informed by the collective set of visions and priorities articulated by the stakeholders. The scenarios range from short-term goals to more ambitious, perhaps less immediately realistic visions. The scenarios fall into five categories.
Homogenous native-plants greening: Under the first set of scenarios, we consider adding native plants homogenously across all block groups, without respect to the specific location. Specifically, we consider two scenarios:
Set 30% of the block group’s area to be as green as native plants, corresponding with a goal set by some scientists and advocates to protect 30% of lands and oceans by 20304;
Set 20% of the block group’s area to be as green as native plants, a level that, according to some of our interviewees is sufficient to support pollinator health.
Equity-driven native plants greening: Under the second set of scenarios, we propose adding native plants to the same amount of area as above (30% or 20% of Denver’s area) but rather than applying the intervention homogeneously across all black groups, we prioritize application of the native plants intervention to those areas of greatest need from a health-equity perspective. Both the State of Colorado and the City of Denver have created equity indices to guide their work. The Colorado Department of Public Health & Environment (CDPHE) describes their equity index here5 and makes the index available for download here.6 The City of Denver Department of Public Health & Environment (DDPHE) describes the neighborhood-level equity index here7 and makes the data available for download here.8 We use both definitions to consider a total of four scenarios:
Set 30% of Denver’s area to be as green as native plants, and apply this intervention to those block groups most in need based on the CDPHE definition of health equity.
Set 20% of Denver’s area to be as green as native plants, and apply this intervention to those block groups most in need based on the CDPHE definition of health equity.
Set 30% of Denver’s area to be as green as native plants, and apply this intervention to those block groups most in need based on the DDPHE definition of health equity.
Set 20% of Denver’s area to be as green as native plants, and apply this intervention to those block groups most in need based on the DDPHE definition of health equity.
Create native buffers around riparian areas (rivers, streams, lakes, and ponds) of the following sizes:
200 feet (ideal for ecosystem health but possibly not realistic)
100 feet (compromise)
50 feet (most realistic; less good for ecosystem health)
Initiatives related to green infrastructure and stormwater management. (Please refer to corresponding section for details.)
Replace a portion of parking-lot surface with native plants:
This section elaborates on the scenarios and visualizes them with maps.
Under the first scenario, which we view as the most ambitious, we consider the expected impact on mortality by applying a green-space intervention to all census block groups, without specifying where.
We first measured the mean NDVI of each census block group on July 4, 2021. The weighted block-group-level mean is presented here, weighted by the proportion each 30 square-meter pixel covers by the census block group. For example, if half of a pixel overlaps the block group, it receives a weight of 0.5 in the weighted average. We also removed bodies of water before measuring NDVI.
Figure 2.1: Scenario 1
For each definition of native-plants NDVI, We excluded census blocks whose baseline NDVI was above the threshold. This map visualizes the NDVI cutoffs both for lower of the two values, that of the Denver Botanic Gardens Green Roof, and the higher of the two, that of Denver Botanic Gardens Native Plot.
Under this set of scenarios, we assumed we had a “budget” of greening of either 30% or 20% of Denver’s area, and we applied that budget to those with most need from a health-equity standpoint based on the CDPHE and DDPHE definitions, described above. As the below maps show, the two health-equity definitions from the State and from the City overlap considerably.
We measured NDVI in a 200-foot buffer, a 100-foot buffer, and a 50-foot buffer around all bodies of water in Denver. We downloaded bodies of water from OpenStreetMap (code here). We defined residential exposure to these riparian areas as those individuals living within a 500-meter buffer, following research on green-space and health that have defined green-space exposure based on residential proximity.3 We estimated the number of people in this buffer by multiplying the population density of the census block group by the intersecting area.
The below map depicts mean NDVI in the portions of census block groups that intersect a 200-foot buffer as well as those pieces that intersect the part of the 500 m buffer that would not be intervened upon, i.e., the part between the 200-foot buffer and the edge of the 500 m buffer.